Technetium is an element whose isotopes are all radioactive. Very small quantities of technetium also exist in nature as the result of spontaneous fission of uranium in uranium ore deposits and neutron capture by molybdenum in molybdenum ore deposits. The vast majority of technetium has been generated synthetically by the fission of uranium in nuclear reactors and is present in significant quantities spent nuclear reactor fuel. The chemical separation of plutonium and uranium from reactor fuel to develop nuclear weapons has generated significant quantities of technetium. For example, at the Hanford site near Richland Washington, technetium is stored in tanks along with other radioactive elements generated as waste a result of plutonium production. The long half-life of technetium-99, its ability to form an anionic species and its volatility make it a major concern for the environment and radioactive waste disposal. Furthermore the migration of technetium away from storage tanks continues to be a significant unresolved problem at several of government storage sites and nuclear actor accident sites including Fukushima, Japan.
Many of the processes designed to isolate and properly dispose of fission products are designed for cationic species like cesium and strontium and are not appropriate for technetium. Technetium, which commonly occurs as the anionic species pertechnetate, is difficult to capture for disposal and because it does not bind to mineral and soil materials, it is very mobile in the natural environment e.g. rivers, lakes streams and groundwater. Technetium is immobile in some environments, particularly in reducing environments such lake bottom sediments, where microbial activity has created reducing conditions. There exists a need for materials and technologies capable of both sorbing or trapping technetium and reducing it in order to prevent its migration.
Various techniques are used to try to isolate leaking storage containers and contaminated soil and sediments to prevent movement of technetium into uncontaminated soil and especially groundwater. As used herein, the term contaminant and contaminated soil will refer to technetium and technetium contaminated soil, respectively.
One approach is simply to attempt to dig up and remove the technetium contaminated soil. This, however, is costly, and disturbance of contaminated soil carries the risk that some technetium will be missed or released and left to migrate further. Excavation also has a negative effect on soil stability. Excessive digging and excavation around waste tanks, for example, has the potential to aggravate technetium transport by damaging heavily corroded containment drums and disturbing already contaminated soil.
Another approach is to establish an impermeable barrier or seal in the soil of a contaminated site in order to prevent migration of contaminants beyond the barriers. Barriers of this sort that are in use at various DOE sites around the United States and abroad include vertical sleeves of steel or plastic placed in trenches surrounding a site. They also include walls formed through the injection of highly pressurized cementatious grout in holes drilled in the soil. Emplacement of such barriers typically causes considerable disturbance to the soil and often there is no convenient way to create a “floor” or continuous barrier beneath the leaking tank or contaminated region. Consequently, the sequestration of the contaminants is incomplete and contaminants continue to migrate downward and may thereafter migrate outward. For areas under waste tanks, waste trenches and certain geological formations, forming a continuous impermeable barrier or seal is difficult and sometimes impossible.
Another approach is to create a permeable, chemically reactive barrier or zone that selectively actively attracts and chemically binds, sorbs, or traps contaminants (i.e., sequestration), while allowing water and other components or contaminants to pass through unaffected.
These chemically reactive materials can be combined with other components to form slurries that harden in the ground, forming semi-permeable reactive barriers. Jet injection processes, for example, are known and used wherein machines pump slurries in holes drilled around the perimeter of a leaking vessel or contaminated site. Additionally, trenches can be dug and backfilled using chemical sorbent materials. Each of these techniques, however, carries the disadvantages previously mentioned relating to significant disturbance of the soil and difficulty in fully surrounding (or encapsulating) a leaking waste tank or region of contaminated soil.
At this time, no chemically reactive material has been proposed or disclosed for the sequestration of technetium.
A need remains, therefore, for a method for sequestering technetium, and in particular, an in situ method of forming a permeable reactive barrier or zone to technetium.